we missed cancelling a reservation with emerald ailse once, when we got home we called to apologize
we asked if we would be charged for the reservation
the reservationist we talked to at the time said the reservation just fell off the system after 12 midnight and that we would not be charged....sure enough there was NO charge to the credit card we had on file...
thus, even though policy says you will be charged, we were not
your mileage may vary, but sometimes "policies" set by rental companies are not followed to the "letter"
we do rent a lot with national so that might make a difference too ( not sure did not ask that question )